The securing together of overlying light gauge metal sheets by means of spot or projection welding is well known. The use of projection welding, which is particularly desirable in many situations, involves the formation of a small raised projection or bead on one of the sheets. A movable weld electrode is pressed against the sheet carrying the projection to cause heating and hence melting of the projection, accompanied by continued pressing of the weld electrode against the sheet to thereby weldably connect the two sheets together. While this projection welding technique is extensively utilized, nevertheless its use in many situations has been prevented or limited in view of the marking or discoloration of the surface of the second sheet in the heat-affected area. Also, many light gauge metal sheets are coated, and most known projection welding techniques have undesirably disrupted or caused significant melting of the coating, such as when the sheets are galvanized or terneplated. Thus, projection welding is not always wholly satisfactory.
To effect projection welding of two overlying sheets, several known welding systems have been utilized for this purpose, which known systems have incorporated a series of capacitors for controlling flow of current to the primary winding of the welding transformer. These systems, commonly referred to as capacitor discharge, spike or ultra-pulse systems, have all utilized capacitors for this purpose. Such systems, however, for the most part have required costly power capacitors, very high voltages on the capacitors and welding transformers, and complex current controls. These overall systems have thus been extremely complex and expensive, and at the same time provide a potentially greater safety hazard in view of the high voltages generated within the systems by the capacitors.
In another known system for projection welding, which system is free of power capacitors, the projection welding is effected by conventional resistance welding techniques. In this apparatus, the welding current is applied to the workpiece over only a fraction of one current cycle so that rapid projection welding can be accomplished. However, because of the speed of welding, the movable welding head (and its associated electrode) must be rapidly moved in response to melting of the projection in order to maintain proper welding pressure between the overlapping sheets. To accomplish this with this known system, the movable welding head has an electromagnet associated therewith so as to create a strong magnetic force, which magnetic force is intermittent and is controlled and synchronized with the weld current so that the movable weld head will rapidly move and maintain proper contact pressure with the sheets to be joined as the projection collapses. This large magnetic force, which is necessary in order to overcome the inertia of the movable weld head, is provided and controlled by means of a second transformer and control provided solely for controlling the magnetic force. This system, due to the dual transformers and controls, is necessarily of substantial complexity and hence is of high cost. Such system, known as a magnetic force welder, has been commercially available for approximately 15 years, although to the best of my knowledge this system has had only limited commercial success.
While other and more conventional resistance welding systems have been utilized for projection welding of overlying sheets, these conventional resistance welding systems have necessarily been limited as to their possible applications, inasmuch as they have been discovered to cause discoloration and surface marking throughout the heat-affected zone, whereby the resulting part is unacceptable in many types of applications. With these conventional resistance welding systems, the main welding transformer is normally connected to the welding head, which movable head is large and has substantial inertia. Further, the secondary of the welding transformer normally has a low voltage and relatively low amperage welding current generated therein. Hence, the timing control for this type system normally causes the welding head to press against the overlying sheets over a substantial period of time, which period of time normally encompasses five to six complete current cycles. This long period of contact is necessary in order for the welding head, due to its high inertia, to respond to the melting of the projection and move so as to maintain pressing engagement with the sheets. In view of this rather long welding contact between the electrode and the sheets, the welding heat affects a substantial area, and hence often causes undesirable discoloration surface marking or melting of coatings.
Accordingly, this invention relates to an improved resistance welding system, specifically for projection welding of light gauge plates or sheets, which system overcomes the disadvantages associated with conventional known systems of the type described above.
More specifically, the improved resistance welding system of this invention incorporates therein a low-inertia movable welding head coupled to a stationary welding transformer which is of low mutual reactance. Through an appropriate and conventional timing circuit and control, a very high-energy pulse of current, of very short time duration, is transmitted through the welding transformer to the movable welding head. The welding head, which has been moved into contact with the sheet carrying the projection so as to activate the welding cycle, hence effects a very rapid and efficient welding of the overlapping sheets or plates, and the welding head rapidly follows up the melting of the projection or bead due to the low inertia of the movable welding head. The system thus provides a desirable weld nugget between the overlapping sheets or plates without creating any significant heat-affected zone, and without causing excessive discoloration or marking of the sheets. This improved system is able to provide for efficient and effective projection welds, while at the same time the system is of minimal cost and is structurally and mechanically simple and compact.
In the improved system of this invention, as briefly described above, the system is totally free of power capacitors, and hence eliminates the use of high voltages and accompanying complex controls required by conventional projection welding systems which rely upon capacitors for developing a current spike or impulse. Further, the improved system of this invention also utilizes only a single transformer, which transformer is only electrically interconnected to the movable welding head which itself is of low inertia, whereby the overall system is hence both structurally and functionally of much greater simplicity than the conventional magnetic force welding system.
The system of this invention is particularly desirable since, in conjunction with its structural and operational simplicity (such as the use of a single transformer in conjunction with a low-inertia movable welding head), it is able to efficiently projection weld two sheets while utilizing a high-energy current pulse of short time duration. Specifically, the welding current pulse occurs over only a small fraction of the total power line frequency cycle. For example, the weld current pulse will never exceed one-half of the power line frequency cycle, and preferably does not exceed one-fourth the power line frequency cycle. Since one 60 hertz current cycle is equal to 16.7 milliseconds, the weld current pulse in the system of this invention thus normally occurs over a time span of three to four milliseconds. Further, the current magnitude in this invention is in the order of five to six times the current magnitude in conventional resistance projection welding.
To provide the system of this invention with a current of magnitude sufficient to create the high-energy short-duration welding current pulse, the welding transformer is designed for low mutual reactance, and further when conventional 460 volt single phase 60 hertz power is supplied to the primary of the transformer, the secondary will have a higher voltage than that for more conventional welding equipment. This combination of low mutual reactance and high secondary voltage generates a steeply rising current of higher magnitude than on conventional equipment to weld the same material thicknesses. This low mutual reactance can be achieved utilizing techniques which are known in the transformer industry, such as by increasing the number of primary and secondary windings so as to minimize the thickness of the individual coils, and by eliminating coil taps on the end coils so that all of the coils can be effectively utilized.
A further advantageous feature of this improved resistance welding system is its capability of carrying out sequential welding operations using current pulses of alternate polarity. The control includes a pair of unidirectional switches which control the current supplied to the primary of the transformer, which switches operate in opposite directions and are alternately sequentially opened by a sequencing device so that sequential welding operations are automatically carried out utilizing high-energy current pulses of alternate polarity. This thus prevents saturation of the transformer core, thereby permitting repetitive and cyclic operations to be carried out at a rather rapid rate over long periods of use.
The system of this invention is thus highly desirable for resistance welding, specifically projection welding or spot welding, relatively light gauge components, such as thin metal components formed from sheets. This improved system has been discovered particularly desirable for use with components in the range of 0.020 to 0.050 inch for permitting fast and efficient projection welding without causing marking or discoloration of the components.
Other objects and purposes of the invention will be apparent to persons familiar with welding systems of this general type upon reading the following specification and inspecting the accompanying drawings .